1. Field of the Invention
This invention relates to an electrophotographic photosensitive member comprising a photoconductive layer made of amorphous silicon which is formed by an electron cyclotron resonance method.
2. Description of the Prior Art
In recent years, as an electrophotographic photosensitive member which is used, for example, in an apparatus for forming images based on electrophotography, there has been proposed a photosensitive member in which a photoconductive layer made of amorphous silicon (referred to as a-Si hereinafter) is formed on a conductive substrate. This a-Si type photosensitive member has numerous advantages such as its long working life, high photosensitivity, high degree of hardness (Hv: 1500-2000 kg/mm.sup.2), and harmlessness to human bodies. Thus, many attempts have been made to put it into practical use as a desirable photosensitive material.
Conventional a-Si type photosensitive members have usually been produced by plasma CVD, sputtering, or other techniques. In the plasma CVD method, a source gas such as monosilane or disilane is first introduced into a vacuum chamber in which a conductive substrate made of aluminum or the like is disposed. The introduction of the source gas into the vacuum chamber is followed by glow discharge with the application of high-frequency power, so that the source gas in the vacuum chamber is decomposed and an a-Si layer containing hydrogen is grown on the substrate. In the sputtering method, which employs a Si wafer as a target, both H.sub.2 gas and a rare gas such as Ar, He, or the like are first introduced into a chamber, and then glow discharge is caused by the application of highfrequency power, so that the target is sputtered and an a-Si layer containing hydrogen is grown on a substrate.
However, to confer sufficient photosensitivity to the a-Si type photosensitive member in the production processes mentioned above, the conductive substrate must be heated to form the a-Si layer thereon. Consequently, the amount of hydrogen contained in the a-Si layer is increased. The excessive amount of hydrogen contained in the a-Si layer makes its electric conductivity as high as 10.sup.-10 s/cm, so that the electric-charge retaining property of the a-Si layer is deteriorated.
The electric conductivity of the a-Si layer can be increased by the addition of boron thereto with the use of, for example, B.sub.2 H.sub.6 gas. In this case, however, the degree of increase is relatively small and there can only be obtained the electric conductivity of at most about 10.sup.-11 -10.sup.-12 s/cm.
The conventional production processes are also disadvantageous in that the deposition rate is very low; the availability of source gas is low; and plenty of powdered polymer such as (SiH.sub.2).sub.n is produced as a by-product and deposited on the surface of the conductive substrate during the growth of a-Si layer, so that many defects can be generated in the a-Si layer, resulting in reduced production yield of a-Si type photosensitive members.
For the conventional a-Si type photosensitive members, the amount of hydrogen contained in the a-Si layer is strictly limited to the range of 10-40 atomic %. (See Japanese Patent Publication No. 60-35059 and U.S. Pat. No. 4,265,991.) Moreover, Japanese Laid-open Patent Publication No. 57-158650 discloses an a-Si layer containing 10-40 atomic % of hydrogen, in which the ratio of the absorption coefficient .alpha.(SiH.sub.2) at around 2100 cm.sup.-1 to the absorption coefficient .alpha.(SiH) at around 2000 cm.sup.-1 in the infrared spectrum of the a-Si layer is in the range of about 0.2-1.7. The absorption coefficient .alpha.(SiH.sub.2) at around 2100 cm.sup.-1 is due to Si-H.sub.2 bonds, and the absorption coefficient .alpha.(SiH) at around 2000 cm.sup.-1 is due to Si-H bonds. To ensure sufficient photosensitivity for the electrophotographic photosensitive members comprising an a-Si photoconductive layer, their resistivity becomes as small as 10.sup.9 .OMEGA..cm, and even when boron (B) is doped in the a-Si layer, their resistivity is still as small as 10.sup.11 .OMEGA..cm, so that the electric-charge retaining property of the a-Si type photosensitive members is inferior to that of conventional selenium or organic photosensitive members.
To improve the electric-charge retaining property of the conventional a-Si photosensitive members, it is necessary to increase the absorption coefficient ratio .alpha.(SiH.sub.2)/.alpha.(SiH). However, in the processes by plasma CVD or sputtering, the reaction of the source gas becomes more active with the increase in high-frequency power in forming each layer, so that a lot of powdered polymer such as (SiH.sub.2).sub.n is produced. The resulting powdered polymer is deposited on the surface of the substrate of the photosensitive member, which impairs the quality of the photosensitive member obtained.